Homosexuality evolved to increase social bonding among humans, University of Portsmouth scientists have said.
Researchers looked at the relationship between progesterone and sexual attitudes in order to explore the role that homosexual behaviour may have played in creating and cementing friendships over the course of human evolution.
The preliminary study, published in the journal Archives of Sexual Behaviour, found that heterosexual women with higher levels of progesterone – the hormone involved in pregnancy and the menstrual cycle – are more likely to be open to the idea of having a lesbian encounter.
Women's levels of progesterone are highest after ovulation, when the chance of falling pregnant is low.
Scientists also found that when heterosexual men are reminded of the importance of having male friends, they have a more positive attitude towards engaging in sexual behaviour with other men.
The latter finding was particularly prominent in men who had higher levels of progesterone.
In a separate study released last week, scientists announced they had found a 'gay gene' that shows sexuality is not a choice. Researchers at the NorthShore Research Institute in the US found a clear link between two specific areas of the human genome and male homosexuality, with lead author Alan Sanders saying their finding "erodes the notion that sexual orientation is a choice".
The University of Portsmouth scientists say that from an evolutionary perspective, many people denounce homosexuality because it serves no reproductive purpose. However, they believe their findings provide evolutionary evidence to support homosexual behaviour.
Progesterone is known to contribute to the formation of social bonds – an adaptive benefit in terms of evolution. It is one of the main hormones responsible for friendly or caring behaviour, and levels rise during friendly interactions.
Researcher Diana Fleischman said: "From an evolutionary perspective we tend to think of sexual behaviour as a means to an end for reproduction. However, because sexual behaviour is intimate and pleasurable, it is also used in many species, including non-human primates, to help form and maintain social bonds. We can all see this in romantic couples who bond by engaging in sexual behaviour even when reproduction is not possible.
"The results of our study are compelling because using two very different methods, they arrived at the same conclusion. Women were more likely to be motivated to think about homosexual sex when their levels of progesterone were higher.
"Compared to a control group, men's homoerotic motivation was not increased by priming them with sex but thinking about friendship and bonding caused a measurable change in their attitude to the idea of having sex with other men."
While having homoerotic thoughts does not necessarily mean people will act upon them, the team say this link may underpin homosexual behaviour.
Researchers asked study participants to complete a survey with questions such as 'The idea of kissing a person of the same sex is sexually arousing to me' and 'If someone of the same sex made a pass at me I would be disgusted', while also measuring their progesterone levels. As progesterone increased, so to did the idea of engaging in homosexual activity.
They added that homosexual behaviour is also seen in other animals, including the great ape family, to forge and maintain friendships.
Fleischman said: "Humans are among a group of animals who have sex for many reasons, not just to reproduce. Reasons can include pleasure, a reward, a way of saying 'please be nice to me' or exerting dominance.
"It's very complex, but it's clear there's a continuum between affection and sexuality and sexuality is fluidity, that is, the ability to engage sexually with those of the same sex or the opposite sex is common. In humans, much, if not most of same-sex sexual behaviour occurs in those who don't identify as homosexual."